Stoichiometry with Other Units

Very few Stoichiometry problems will ever ask you to convert between moles of one substance and moles of another, as we did on this page.

Almost always, you will be asked to convert from a measured amount of one thing (grams, molecules, mL of solution, L of gas, etc) to another measured unit of something else. This addition can make stoichiometry problems seem harder, but it doesn’t. What it does is make them LONGER, but the work is the same as what you’ve been doing all along.

Let’s look at a sample problem and pull it apart:

How many molecules of water are produced when 13.8 g of butane burn?

Here are the things you should notice:

1) The problem mentions a reaction.

How many molecules of water are produced when 13.8 g of butane burn?

That implies it is a stoichiometry problem.

2) The problem gives an amount of one compound and asks for an amount of another compound.

How many molecules of water are produced when 13.8 g of butane burn?

Now we KNOW that it is a stoichiometry problem.

3) The problem deals with units OTHER than moles.

How many molecules of water are produced when 13.8 grams of butane burn?

That means we’ll have to do more than just the reaction fraction (although that is still the heart of the problem).

Let's Solve It

Here's the problem again:

How many molecules of water are produced when 13.8 g of butane burn?

There are LOTS of ways to organize yourself, but I think that the most effective way is to balance the reaction and then write the information you know UNDER each part of the reaction, like this:

This problem will look like lots of other problems we've done (with the given fact on the left and the needed amount on the right and fraction(s) between. The only question is how many fractions? The answer is easy. Any stoichiometry problem that involves units other than moles requires three fractions. (see #1-3 above).

So, we'll start the problem like this:

The middle step is the reaction fraction. (This is the fraction that changes from one compound to another.)

Since the reaction fraction is in moles, we'll have to use the first fraction to change from g of butane to moles of butane

If we cancel units (so far) we will recognize that, if we solved the problem now, we would have moles of water.

Given that we were asked for molecules of water, we know what to do with the last fraction.

Now, we can solve it

If you look closely at the three fractions, you will see the pattern that is present in ALL stoichiometry problems. The three steps are always:

1. Go to moles
2. Use the reaction fraction
3. Go to what you want

Problems can look a little different with different units, but in the end, the process always involves those three steps!

Let's take a look at two more problems, one with molarity and one with gases.